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Silvio

Simple soft start for high power transformer

Here is a simple soft start which was needed to power up my large toroidial transformer (2KW). The transformer induces a large surge on switch on and at times it was tripping my 32 amp MCB. However here I am sharing with you what I did to cure this problem. The delay of the soft start is around 1 second but however the timing can be adjusted to one's need by changing the value of the resistor connected to the base of the transistor. The timing can also be changed by changing the value of the capacitor attached with the base resistor.

Circuit description.
The circuit is rather strait forward and this consists of an X2 capacitor with a value of 0.33 uF rated at 250 volts. This cap is placed in series with the input, followed by a small bridge rectifier and smoothing capacitor This will reduce the mains voltage to the desired level. However the value of the capacitor was found by trial and error as it all depends on the current of the relay and voltage needed to energize it. However my choice was with a 48vdc relay coil, as this consumes less current than a lower coil voltage relay with the same capacity. My result was that I had around 50 volts dc at 230 volts ac input.
The rectified voltage is then fed via a 130K ohm resistor feeding the base of Q1 and a 1000uf capacitor which has the negative tied to the negative side of the dc voltage line. This capacitor will charge up slowly and when the threshold voltage is reached (0.7v) Q1 comes on and energize the relay coil thus shorting the current limiting resistor. A 4.7K ohm resistor is tied across the timing capacitor so that it discharges it when the supply is switched off. However around 15 seconds have to be paused between each start up to allow ample time for the timing capacitor to discharge so that the full delay time can be achieved. The circuit was built with minimum component count in mind. One can always hook up a 555 timer chip but this will add up to more components such as having to insert also a voltage regulator and making the circuit more complex.